I'm using Mac High Sierra. Unfortunately I'm not able to upgrade it any more.
Hmm, actually that shouldn't be the problem, that version should be supported. I'll look into it when I have time (later this week probably), it might be trickier to figure out.
Could you maybe fill in this form please? (You can just put in your email, I haven't set it up properly yet). I haven't got a computer with an environment like yours so it's hard for me to test whether it would work or not, I might need to send a build to you to test if it works. (Hopefully that won't be necessary, but just in case)